1. Field of the Invention
The present invention relates to a through-the-lens system for measuring the height of a sample when neither the lens nor the sample is moved. More particularly, the invention relates to a through-the-lens system for measuring two or more heights within the sample simultaneously; for example, when a transparent surface is disposed above another surface, the height differential between the two surfaces can be measured.
2. Description of the Related Art
Prior-art optical sensors have used a number of various techniques including parallax, time elapse, triangulation, trilateration, autofocusing, stadimetry, and interferometry.
U.S. Pat. No. 4,204,772, issued to N. Balasubramanian, discloses an optical measuring system which measures height differences on a surface. A beam of light is focused by a lens onto a surface. The beam is not perpendicular, but is at an angle to the surface. If the surface moves in a normal direction, the point illuminated by the beam is displaced laterally due to the inclination of the beam. The displacement is detected by a sensor and measured by physically shifting the lens and correlating this movement to that of the surface.
Nakagawa et al., in U.S. Pat. No. 5,151,609, disclose an apparatus for detecting solid shapes. They use computer technology, operating on a series of images, to generate the shape. The object to be modelled is mounted on a z-axis table which slowly raises the object while it is imaged with a TV camera. The surface of the object must be sufficiently rough (e.g., a sintered surface) to show grain in the image at the z-elevation where the camera lens is focused. The remainder of the surface will be out of focus, and will not show grain in the image. The images are sent through a contrast extraction circuit and other electronic processes to generate an outline for each z-level. A three-dimensional model is constructed from the outlines.
German Patent Publication No. 2,741,807, published Mar. 29, 1979, shows a device in which a collimated beam of light passes through a beam splitter and is focused through a lens onto a surface, the position of which is to be measured. The reflected light from the beam passes back through the lens and is diverted by the beam splitter to a second lens which focuses onto a pinhole to a photo-receiver. The lens is oscillated back and forth. The lens oscillator and the photo-receiver are both connected to a memory and recording device. The device is limited in speed of operation by the motion of the lens.
German Patent Publication No. 3,528,684, published Mar. 6, 1986, depicts a variable-focus system for measuring surface heights. This system has a moving lens driven back and forth along the optical axis.
German Patent Publication No. 3,817,337, published Dec. 1, 1988, shows an optical surface profiler that includes a driver for moving the sample or measured object along the optical axis, along with a variable deflection device and a detector.
International publication number WO 88/07657, published Oct. 6, 1988, shows a distance measuring device using a special lens which focuses a beam into a linear focus, rather than the usual point focus. The reflected beam is diverted by a beam splitter and analyzed by a detector that measures spatial distribution of the reflected light.
Japanese Patent Publication No. 63-229,314, published Sep. 26, 1988, teaches the use of double photo-detectors to measure the inclination of a surface. The invention has a 1/4-wave plate and beam splitters.
Carl Zanoni, in U.S. Pat. No. 3,768,910, discloses a system using focus modulation to detect the position and motion of a test surface. A beam of light is sent through a first lens onto a mirror which is mounted on a motion translator (transducer). The mirror oscillates sinusoidally back and forth around the focus point of the first lens. A beam reflected by the mirror is diverted by a beam splitter toward a second lens which focuses the beam onto the test surface. Light reflected from the test surface returns through the second lens and is diverted by a second beam splitter to a third lens which focuses the reflected light through an aperture onto a photodetector. The photodetector produces amplitude and phase signals that change with the position of the surface. Based on this information, a rectifier drives the mirror such that the focal plane of the beam is moved toward the sample surface. Thus, Zanoni uses a feedback loop to maintain the focal plane at the sample surface. The device disclosed in the Zanoni reference cannot effectively measure the distance between two surfaces disposed axially along the same optical axis (such as the partially-reflective surface of a transparent layer disposed above an underlying metallic layer) because the feedback loop would cause the focus to be driven toward only one of the surfaces.